The yarn tensioner known from U.S. Pat. No. 5,979,810 A (DE 195 31 579 B1) comprises disc-shaped tensioning elements. The first tensioning element is pressed by the second tensioning element by the adjustable magnet pressing force against a stationary stop. The repelling magnet is provided at the rear side of the second tensioning element remote from the first tensioning element and actuates the magnet armature which is arranged in the second tensioning element. The magnet pressing force can be varied steplessly while the thread is running. In case of a slub or a knot occurring in the thread the mass of the second tensioning element must be pressed away together with the mass of the magnet armature and counter to the repelling magnet force of the magnet and away from the first tensioning element which is supported at the stationary stop. Due to the inertia of the large mass, specifically of the magnet armature, a momentary thread tension rise occurs which may lead to a rupture of the thread.
In the thread tensioner known from U.S. Pat. No. 6,161,595 A the first tensioning element is provided at a stationary magnet body. The second tensioning element is movable in relation to the first tensioning element and is actuated by a magnet generating a pulling magnet force through the first tensioning element. In case of a passage of a knot in the thread the second tensioning element is moved counter to the magnet force away from the first tensioning element, whereby the gap width, which is decisive for the strength of the magnet force, is varied, even when the second tensioning element only tilts sidewardly. This momentary increase of the gap width reduces the magnet force significantly such that the tensioning effect is decreased and so that the second tensioning element returns with a critical oscillating phase after the passage of the knot and with a relative delay to the home position. In case of thick thread material the return motion takes place very slowly and in connection with a significant oscillating phase.
In the thread tensioner known from WO 03/033385 A the first tensioning element is provided at a stationary magnet body. The second tensioning element is movably held in a tiltable lid which grips over the magnet body. The second tensioning element is actuated through the first tensioning element by a pulling magnet force and is pressed against the first tensioning element. In case of the passage of a slub or a knot in the thread the second tensioning element is lifted counter to the pulling magnet force such that the strength of the magnet force is reduced and such that the tensioning effect is changed. Specifically in case of a thick thread material the return movement of the second tensioning element after the passage of a knot or a slub may either be delayed or occur with an oscillation phase during which the tensioning effect varies.
It is an object of the invention to provide a thread tensioner of the kind as disclosed above which allows the passage of slubs and knots in the thread without danger to the thread, which then does not vary the tensioning effect significantly, and which instantaneously adjusts the original tensioning effect after the passage of the knot or the slub. The thread tension ought to be useful, in particular, for thick thread qualities.
The function of the thread tensioner according to the invention considers the phenomena that a knot (or a slub) which passes the thread tensioning zone in the running thread with relatively high speed is generating a momentary energy impact lateral to the thread running direction which energy impact has a relatively high frequency. Either the first tensioning element is responding to the occurrence of the energy impact by yielding counter to the spring force, while the second tensioning element and the mass of the magnet armature do not react significantly due to inertia, or the second tensioning element is yielding counter to the spring force, while the magnet armature does not react significantly thanks to the large mass. In each case it is assured that the tensioning effect is not significantly reduced while the knot is passing, because the originally set magnet pressing force or the spring force, respectively, is maintained substantially without any reduction. Furthermore, the deflected tensioning element returns after the passage of the knot instantaneously and without an oscillating phase to the home position, since the tensioning element permanently is under the unchanged force action of the spring force. The thread tensioner having this structure is useful for practically all thread qualities with the same advantages, however, specifically for thick thread material, which generates upon the passage of a knot or a slub a considerable lifting motion. The mass of the respective tensioning element is selected so that the mass can be displaced by the energy impact generated by the knot while the substantially larger mass of the magnet armature will not be displaced by the influence of this energy impact.
The mass of the first tensioning element is displaced counter to the spring force when a knot occurs, while the magnet armature together with the second tensioning element remains substantially motionless. During normal tensioning of the thread without a knot or a slub occurring in the thread the first tensioning element will remain with the spring force at the stationary stop such that the first tensioning element acts like a stationary tensioning surface for the second tensioning element.
The spring assembly provided between the second tensioning element and the magnet armature defines a coupled arrangement for the masses such that the second tensioning element is displaced by a knot counter to the spring force and relative to the magnet armature while the magnet armature remains substantially motionless.
In both cases the originally adjusted tensioning effect is not changed upon passage of a knot. Furthermore, returns the displaced tensioning element immediately into the home position since the displaced tensioning element remains loaded by the in some case even increased spring force or the spring force and the adjusted magnet pressing force.
Expediently, the thread tensioner is a controlled leaf spring tensioner in which the first tensioning element is a leaf spring, while the second tensioning element is a body forming a tensioning surface.
In this case there even may be provided another type of a controlled thread tensioner the first and/or second tensioning element of which is not a leaf spring but e.g. is a rigid body instead.
The leaf spring expediently has the shape of a J with a freely cantilevering end and is anchored with the J-hook to a, preferably, rotatably adjustable support. The spring force is generated by the support, by which force the leaf spring is pressed against the stationary stop such that the leaf spring behaves during normal tensioning operations like a stationary tensioning surface or does not significantly leave the stationary stop even when the magnet pressing force is adjusted to a maximum. By means of a rotatably adjustable support the acting spring force e.g. can be adjusted arbitrarily upon demand.
The second tensioning element expediently is a U-shaped body which either is rigid or resilient, e.g. a leaf spring body which is movably held in a guidance substantially in the direction of the adjustable magnet pressing force. The guidance positions the body in relation to the leaf spring and so that the adjusted magnet pressing force comes into action in the tensioning zone as desired. Furthermore, the guidance allows an easy replacement of the second tensioning element.
In an expedient embodiment the leaf spring (the first tensioning element) is broader at least in the region of the stationary stop than the body (the second tensioning element) which forms the tensioning surface. The leaf spring is supported at the stationary stop by edge regions which protrude sidewardly beyond the body.
The repelling magnet actuator expediently comprises a proportional electromagnet coil which is connected to a current control. In this fashion it is possible to adjust the tensioning force of the magnet armature, e.g. of a permanent magnet, extremely rapidly and delicately, for example, when using the thread tensioner between a thread feeding device and a shuttleless weaving machine in which relatively high thread speeds occur and where a thread tension is desirable which is as uniform as possible and which has to be changed, in some cases several times, within an insertion process. The magnet pressing force directly depends on the strength of the actuating current of the coil.
In a preferred embodiment a stable support of the leaf spring is achieved by ribs for both edge regions of the leaf spring which ribs are provided at both sides of the body.
In a particularly preferred embodiment two thread tensioners are provided on a common carrier and substantially reversed left to right, preferably with an offset in thread running direction. This thread tensioner device is of compact size and can be used for processing two threads which run close to one another. However, each thread tensioner can be controlled individually.
In a structurally simple, reliable and compact embodiment the body forming the tensioning surface is arranged on a disc, preferably with a resilient member between the body and the disc. The disc is coupled via a connection with the magnet armature, preferably with a permanent magnet. In this case the magnet armature is guided together with the disc in an axial guidance so that the magnet armature transmits the magnet pressing force smoothly and so that the disc actuates the second tensioning element in centered fashion.
The axial guidance, in a preferred embodiment, is held in a housing of the magnet actuator.
The ribs defining the stationary stop for the first tensioning element may expediently also be provided at the housing, preferably even in unitary fashion.
The connection having the task of the guidance and the task of the transmission of the force may comprise a guiding body at which the disc is held via a fastening element and an axially and radially compressed O-ring. The guiding body may have a long guiding surface serving as an axial guidance. The compressed O-ring has a centering function and generates a desirable elasticity within the connection.
Since such a thread tensioner expediently operates with a low basic tensioning effect as long as the coil is not supplied with current, it is expedient to place a stationary auxiliary permanent magnet in alignment with and in axial distance from the magnet armature, which auxiliary permanent magnet has a polarisation which is opposite to the polarisation of the magnet armature and which actuates the magnet armature permanently and repellingly. Instead of such a permanent magnet alternatively a weak spring could be provided, the spring force of which may be adjustable.